A toy-building element

ABSTRACT

A toy-building element for a toy-building system in which toy-building elements can be releasably coupled to one another using an interference fit between coupling studs of one toy-building element inserted in matching openings in another toy-building element. The toy-building element includes a body part with a top face, a bottom face and one or more flat sidewalls that extend around an outer periphery of the toy-building element, the one or more flat sidewalls at least partially define the interior of the toy-building element and the flat sidewalls extend between the top face and the bottom face with the sidewalls being connected to one another at right angles to form sharp corners, at least one cylindrical coupling stud extending at a given height from the top face, and at least one tubular projection extending inside the interior to the bottom face for forming at least one opening in the bottom face in which a coupling stud fits with the interference fit. The at least one projection is detached from the flat sidewalls. The at least one tubular projection includes four substantially flat contact walls extending perpendicularly to the bottom face, the four contact walls being arranged in a squared arrangement with each of the four contact walls arranged at a 45° angle with the side walls.

TECHNICAL FIELD

The disclosure relates to a toy-building element for a toy-building system in which toy-building elements can be releasably coupled to one another using an interference fit between coupling studs of one toy-building element inserted in matching openings in another toy-building element.

BACKGROUND

Lego Group manufactures and sells Lego® toy-building system sets in which toy-building elements can be releasably coupled to one another using an interference fit between coupling studs of one toy-building element inserted in matching openings in another toy-building element.

Most Lego® toy-building elements (pieces) have two basic components: coupling studs on top and tubes on the inside extending downwards from the top. A piece's coupling studs are slightly bigger than the space between the tubes and the walls or projections from the walls. When pieces are pressed together, the coupling studs push the walls out and the tubes in. The material is resilient and tries to retain its original shape, so the walls and tubes press back against the coupling studs. In the process, the coupling studs will also slightly deform, but due to the high rigidity of the studs this deformation is small compared to the pushing out of the walls and pushing in of the tubes. Friction prevents the two pieces from sliding apart. This stud-and-tube coupling system uses an interference fit, a firm, friction-based connection between two parts without the use of an additional fastener.

Cylindrical coupling studs extend at a given height above the top portion of the toy-building element. Inner tubes and inner ribs are provided for engaging the coupling studs with an interference fit that allows the bricks to be releasably coupled to one another.

Basic Lego® building pieces are e.g. 2×2 plates and 2×4 plates, 2×2 bricks and 2×4 bricks, but this list is not exhaustive and many other variations in size and shape exist.

Lego® toy-building elements start out as plastic granules composed primarily of acrylonitrile butadiene styrene. An automated injection molding process turns these granules into toy-building elements such as e.g. bricks and plates. The making of a Lego® toy-building element requires high temperatures and large powerful injection molding equipment. The injection molding equipment melts the granules at temperatures of up to 232 degrees C., inject the melted ABS into molds and applies between 25 and 150 tons of pressure. After about seven seconds, the new toy-building elements cool and fall onto a conveyor. At the end of the conveyor, they fall into a bin.

A major cost factor in the production process of such toy-building elements is the fact that they have to be manufactured with a relatively high precision and with low tolerances in order to have a reliable interference fit between two toy-building elements, i.e. in order to ensure that the interference fit engages and disengages with the same force, and stays the same within limits, also for older used toy-building elements, and under different circumstances, such as e.g. temperature and different number of studs that are engaged between two toy-building elements.

Production of components is subject to variation. The component is designed with a certain tolerance for each dimension. The tolerance is a specification of how large a variation that is permissible on the given dimension. Components with tight tolerances are generally more expensive to produce. For injection-molded components this is an effect of higher precision of molds required, longer cycle times and in general a more sophisticated production technique.

Mechanical Interfaces are often designed as press fits where a geometric overlap between the parts ensures contact forces between the parts. In such Mechanical Interfaces, a small variation in overlap can cause a large variation in contact force. The relation is defined by the stiffness of the components.

Toy-building elements, such as e.g. from the Lego® toy-building system, require very high precision production since a small variation in overlap causes a large variation in contact force.

WO2014009345 discloses a toy-building element according to the preamble of claim 1.

GB935308 discloses a toy building block comprises a hollow block open at one face and having on the closed face opposite the open face externally projecting assembly studs, and having within the cavity of the block a rectangular grid of ribs to provide faces for engaging the assembly studs of an adjacent block, the ribs being disposed at an angle of 45 degrees to the end and side walls of the block and the distance between two adjacent parallel ribs being such that the ribs would engage snugly an assembly stud of a like juxtaposed building element. The rectangular grid of ribs is attached to the end and sidewalls thereby increasing the overall stiffness of the toy building block and the rectangular grid of ribs.

WO9811968 discloses a toy building set comprises building elements of a first type with coupling studs arranged in a two-dimensional periodical pattern with the coupling studs disposed in rows in two main directions perpendicular to each other in such a manner that the coupling studs also form diagonal rows in diagonal directions relative to the main directions, and building elements of a second type for interconnecting with building elements of the first type. The building elements of the second type having pairs of parallel coupling walls which define cavities with coupling means for receiving coupling studs on building elements of the first type in a releasable engagement in such a manner that the coupling walls are arranged in main directions between rows of coupling studs. The building set further comprises building elements of a third type for interconnecting with building elements of the first type, the building elements of the first type having pairs of parallel coupling skirts that define cavities with coupling means for receiving a plurality of coupling studs on other building elements in releasable engagement. The coupling skirts are arranged in diagonal directions in spaces between diagonal rows of coupling studs.

It is an object to provide a toy-building element that is less sensitive to production tolerances while maintaining the same reliability of interference fit. Alternatively, it is an object to provide a toy-building element that allows larger production tolerances while maintaining the same reliability of interference fit or to provide a toy-building element that with current production tolerances increases the reliability of interference fit.

SUMMARY

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description and the figures.

The inventors arrived at the insight that the need for very high precision during production can be reduced by designing the mechanical interfaces more flexible, thereby reducing sensitivity towards variation caused by production tolerances. The inventors also arrived at the insight that reduced stiffness of the parts can be compensated by designing a larger overlap, such that the target for the interface force can be achieved.

A mechanical interface with large overlap and small stiffness will be less sensitive to variation in overlap than an interface with small overlap and large stiffness. Therefore, the toy-building elements can be produced with larger tolerances while maintaining a similar variation in force of the interference fit, both for engaging and for disengaging.

Alternatively, the tolerances can be maintained and the variation in force will then be decreased.

According to a first aspect, there is provided a toy-building element for a toy-building system in which toy-building elements can be releasably coupled to one another using an interference fit between coupling studs of one toy-building element inserted in matching openings in another toy-building element, the toy-building element comprising:

-   -   a body part with a top face, a bottom face and one or more flat         sidewalls that extend around an outer periphery of the         toy-building element,     -   the one or more flat sidewalls at least partially define the         interior of the toy-building element and the flat sidewalls         extend between the top face and the bottom face with the         sidewalls being connected to one another at right angles to form         sharp corners,     -   at least one cylindrical coupling stud extending at a given         height from the top face, and     -   at least one tubular projection extending inside the interior to         the bottom face for forming at least one opening in the bottom         face in which a coupling stud fits with the interference fit,         the at least one projection being detached from the flat         sidewalls,     -   wherein the sides of the sidewalls facing the interior (19) can         be provided with contact ribs that extend from the bottom face         towards the top face,     -   wherein at least one tubular projection comprises four         substantially flat contact walls extending perpendicularly to         the bottom face, the four contact walls being arranged in a         squared arrangement with each of the four contact walls arranged         at a 45° angle with the side walls, for increasing the         flexibility of the interference fit, the four contact walls of a         tubular projection are connected to one another to form a tube         with a squared cross-sectional outline, and in that     -   a contact wall forms together with two sidewalls or with two         contact ribs an opening in which a coupling stud fits with an         interference fit with three contact lines and/or wherein a         contact wall forms together with one sidewall or one contact rib         and one contact wall of another tubular projection an opening in         which a coupling stud fits with an interference fit with three         contact lines.

The inventive insight on which the invention is based is to design the mechanical interfaces between the toy-building elements such that they are as flexible as possible while still being able to withstand the loads in the interface.

By placing the contact walls in a squared arrangement with the contact walls at 45° angles to the sidewalls and in a way that the contact line between a stud and the contact wall concerned is in the middle of the contact wall, the contact wall becomes a “beam” with the load in the middle. Thus, the stud engages the contact wall at the point where it is most flexible, making the interference fit less sensitive to production tolerances.

In the toy-building element disclosed in WO2014009345, the projection is a tube with an annular cross-section with the outer surface of the tube being in the form of a circular cylinder. A tubular projection with such a shape is very stiff i.e. not flexible. The outer surface of the tube at a contact line with a stud is convex and the overall shape of the tube results in a non-flexible engagement object. The same applies to the studs, which are also in the form of a tube with an annular cross-section and an outer surface in the form of a circular cylinder. Thus, in the prior art two very stiff objects engage one another and consequently production type tolerances need to be very tight, thereby increasing production costs. In the prior art and in the present toy-building element, there are always three contact lines between the studs of one brick and the inner side of another brick. At least one contact line is formed by an engagement between the projection and a stud and at least one contact line is formed by an engagement between a sidewall, or a rip projecting from the sidewall and the stud. The third contact line can be either between the stud and another projection or between the stud and another side wall. Two of the contact lines are spaced at 90° around the stud while the other contact line is spaced 135° from the first two contact lines.

In a first possible implementation of the first aspect, at least a portion of the contact walls is substantially flat, i.e. planar.

In a second possible implementation of the first aspect, the thickness of the contact walls follows the distribution of the bending moment applied to the contact wall when a stud is inserted.

In a third possible implementation of the first aspect, the four contact walls of one tubular projection together define an opening in which a coupling stud fits with an interference fit.

In a sixth possible implementation of the first aspect, contact lines that are not associated with a contact wall coincide with the ribs, the contact lines extend preferably parallel with the axis of the coupling studs and extend even more preferably over the height.

In a seventh possible implementation of the first aspect, two of the three contact lines are angularly spaced by a 90° angle from one another around the coupling stud with the third contact line being spaced by a 135° angle from each of the other two contact lines.

In an eighth possible implementation of the first aspect, four contact walls of a tubular projection are spaced from one another by a gap between the contact walls.

In a ninth possible implementation of the first aspect, the four contact walls of a tubular projection are connected to one another from a tube with a squared cross-sectional outline.

In a tenth possible implementation of the first aspect, the cross-sectional outline of the tube resembles or equals a rounded square, a square with rounded corners or a squircle.

In an eleventh possible implementation of the first aspect, the toy-building element is provided with at least two of the tubes with neighboring tubes being interconnected with one another at opposing corners of the tubes concerned.

In a twelfth possible implementation of the first aspect, the wall thickness of the tubes concerned at the opposing corners is reduced relative to the wall thickness of the other corners of the tubes concerned.

In a thirteenth possible implementation of the first aspect, the wall thickness of the tube is preferably substantially constant in the direction of the length of the tube and is varied along the circumference of the tube.

In a fourteenth possible implementation of the first aspect, the wall thickness of the tube is varied along the circumference of the tube in order to obtain a homogeneously distributed stress level in the tube when a stud is disposed in an opening bordering the tube concerned or in the tube concerned.

In a fifteenth possible implementation of the first aspect, the wall thickness of the tube is varied along the circumference of the tube in order to obtain maximum resiliency, i.e. maximum flexibility in relation to non-permanent deformation by the insertion of a stud.

In a sixteenth possible implementation of the first aspect, the wall thickness along the circumference of the tube is higher at the corners of the tube and higher in the middle of the extent of the tube between corners and lower in the remaining circumferential extent of the tube.

In a seventeenth possible implementation of the first aspect, the at least one cylindrical coupling stud having a given outer diameter, and wherein the size of the gap between opposing contact walls of a tubular projection is slightly less than the diameter.

In an eighteenth possible implementation of the first aspect, the circular outline of the coupling studs is slightly larger than the opening between the contact walls and the side walls or the ribs on the inner side of the side walls.

In a nineteenth possible implementation of the first aspect, the top face is provided with at least two rows of equidistantly spaced coupling studs in a squared pattern.

In a twentieth possible implementation of the first aspect, the openings defined between the contact walls and the side walls or with ribs on the side of the sidewalls facing the interior are arranged in a pattern that matches the squared pattern.

In a twenty-first possible implementation of the first aspect, the flat side walls meet one another at sharp corners.

In a twenty-second possible implementation of the first aspect, the pitch between the coupling studs in the squared pattern is 8 mm.

In a twenty-third possible implementation of the first aspect, the height of a building element between the bottom face and the top face is 9.6 mm.

In a twenty-fourth possible implementation of the first aspect, the length of a building element with a rectangular top face and bottom face outline is equal to the product of the number of coupling studs along the length of the building element times 8 mm, with 0.2 mm subtracted from the product to arrive at the length.

In a twenty-fifth possible implementation of the first aspect, the width of a building element with a rectangular top face and bottom face outline is equal to the product of the number of coupling studs along the width of the building element times 8 mm, with 0.2 mm subtracted from the product to arrive at the width.

In a twenty-sixth possible implementation of the first aspect, the coupling studs have a diameter D of 4.9 mm.

In a twenty-seventh possible implementation of the first aspect, a contact wall is provided with a contact rib extending from the end of the contact wall at the bottom face towards the top face, a contact rib on a contact wall preferably being arranged to form the contact surface with a stud from another toy-building element.

In a twenty-eighth possible implementation of the first aspect, the coupling studs having a continuous abutment face shaped as a cylinder face whose generatrices extend approximately at right angles from the top face of the body part to the top of the coupling stud.

In a twenty-ninth possible implementation of the first aspect, the toy-building element constitutes a toy-building brick.

According to a second aspect, there is provided a method for manufacturing a toy-building element according to one or more of the foregoing aspects and possible implementations thereof, wherein the method includes injection molding the toy-building element in a mold that comprises at least two mold parts, where one of the mold parts comprises a mold core, which shapes the inner faces of the toy-building element, wherein the mold core comprises a recess to form the at least one tubular projection.

According to a third aspect, there is provided a molding tool for use in manufacture of a toy-building element according to the first aspect or any implementations thereof, wherein the mold comprises at least two mold parts, where one of the mold parts comprises a mold core, which shapes the inner faces of the toy-building element, and wherein the mold core comprises a recess to form the at least one tubular projection.

These and other aspects and implementations of the invention will be apparent from the embodiment(s) described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following detailed portion of the present disclosure, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which:

FIG. 1 is an elevated view showing a toy-building element according to an example embodiment including its top face,

FIG. 2 is an elevated view of the toy-building element of FIG. 1 showing its bottom face,

FIG. 3 is a bottom view of the toy-building element of FIG. 1 with a coupling start of another toy-building element inserted in an opening in the bottom face,

FIG. 4 is a bottom view of the toy-building element of FIG. 3 with a coupling start of another toy-building element inserted in another opening in the bottom face,

FIG. 5 is an elevated view showing the toy-building element according to another example embodiment showing its top face,

FIG. 6 is an elevated view of the toy-building element of FIG. 5, showing its bottom face,

FIG. 7 is a bottom view of the toy-building element of FIG. 5,

FIG. 8 is a detail of a toy-building element according to FIG. 1 or FIG. 5,

FIG. 9 is a detailed elevated view of the bottom face of another embodiment, and

FIG. 10 is a detailed bottom view of the toy-building element of FIG. 9.

DETAILED DESCRIPTION

FIGS. 1 to 4 illustrate a toy-building element according to a first example embodiment in various views. The toy-building element 10 is part of a toy-building system in which toy-building elements 10 can be releasably coupled to one another using an interference fit between coupling studs 15 of one toy-building element 10 inserted in matching openings in another toy-building element 10. The toy-building elements 10 can all have identical shapes but also toy-building elements with different shapes can be assembled. The toy-building element is produced by injection molding in a mold with at least two mold parts.

The toy-building element 10 has a body part with a top face 13, a bottom face 12 and one or more sidewalls 14 that extend around the outer periphery of the toy-building element 10. The top face 13 opposes the bottom face 12 and the sidewalls 14 connect the top face 13 to the bottom face 12. The sidewalls 14 are connected to one another at right angles and form 90° corners with sharp edges.

The toy-building element in the embodiment shown in FIGS. 1 to 4 is a toy-building brick with a cuboid body.

The side faces 14 extend over a height H between the top face and the bottom face, and the sides faces 14 have a length L along a longer side of the toy-building element 10 and have a width W along the shorter side of the toy-building element 10.

The top face 13 of the toy-building element 10 according to the present embodiment is provided with two by four rows of equidistantly spaced coupling studs 15 in a squared pattern. The inner surface of the sidewalls 14 is provided with a plurality of ribs 16 that each extend from the bottom face towards the top face 13. The ribs 16 protrude from the inner surface of the sidewalls.

Two ribs 16 are located near each corner of the body of the toy-building element 10. In the toy-building element 10 according to the present embodiment, there are additional ribs 16 that are provided on the inner side of the longer sidewalls 14 on an imaginary line that extends at a right angle to the longer sidewalls 14 and through in the middle between the centers of two neighboring projections 20.

The flat sidewalls 14 at least partially define the interior 19 of the toy-building element 10. The toy-building element 10 according to the present embodiment is provided with three tubular projections 20 extending inside the interior 19. The tubular projections 20 extend from the top face 13 towards the bottom face 12. The tubular projections 20 extend in an embodiment all the way to the bottom face 12 so that the free end of the tubular projections 20 is flush with the bottom face 12. In another embodiment, the projections 20 extend almost all the way to the bottom face 12.

The tubular projection 20 comprises four substantially flat contact walls 22 extending perpendicularly from the top face 13 to the bottom face 12. The four contact walls 22 are arranged in a squared arrangement with each of the four contact walls 22 arranged at 45° angles with the side walls 14. Thus, two pairs of the contact walls 22 are opposing one another, with each of the pairs being at a 90° angle with the other pair.

In the present embodiment, the four contact walls 22 of a tubular projection 20 are connected to one another to form a tube with a squared cross-sectional outline. The cross-sectional outline of the tube resembles or equals a rounded square, a square with rounded corners or a squircle. In the present embodiment, the toy-building element 10 is provided with three tubes with neighboring tubes being interconnected with one another at opposing corners of the tubes concerned. The connection between neighboring tubes is formed by a narrow bridge 28 formed by reducing the thickness of the tubes at neighboring corners 24 so that the wall thickness at the connection between the tubes does not become higher than necessary and possible with respect to the injection molding process for producing the toy-building elements 10. Cutouts 29 are the result of the reduction in wall thickness at the meeting corners 24. The wall thickness of the tubes concerned at the opposing corners 24 is reduced relative to the wall thickness of the other corners 24 of the tubes concerned.

The wall thickness of the tube is in an embodiment reduced in the direction of the length of the tube seen from the top face 13 to the bottom face 12 to provide draft for allowing the toy-building element 10 to be removed from the mold at the end of the injection molding process. The wall thickness of the sidewalls 14 is reduced accordingly in the direction from the top face to the bottom face in order to provide draft for allowing the toy-building element 10 to be removed from the mold. The draft of parts of the toy-building element 10 is best shown in FIG. 10.

The wall thickness of the tube is in an embodiment varied along the circumference of the tube with the wall thickness of the tube being higher at the corners 24 of the tube and higher in the middle 26 of the extent of the tube between corners 26 and lower in the remaining circumferential extent of the tube 25, as illustrated in FIG. 8. Thus, the wall thickness of the tube is lowest in the area between the center 26 of a contact wall and a corner 24. This provides for an equal distribution of the stress upon the middle 26 of the extent of the tube being pressed by a stud being inserted in an opening adjacent the contact wall 22 concerned.

The projections/tubes 20 are sized and shaped for forming an opening in the bottom face 12 in which a coupling stud 15 fits with an interference fit. The opening with the correct size for an the interference fit with a stud 15 of another building element 10 is formed by a contact wall 22 together with two ribs 16 associated with a corner of the toy-building element 10, as shown in the upper portion of FIG. 4, or the opening with the correct size for an the interference fit with a stud 15 of another building element 10 is formed together with two ribs 16 associated with a corner of the toy-building element 10, as shown in the lower portion of FIG. 4.

The four contact walls 22 of one projection 20 together define an opening in which a coupling stud 15 fits with an interference fit with four contact lines, as shown in the upper portion of FIG. 3. Hereto, the distance d between the surfaces of opposing contact walls 22 is selected to be slightly less than the diameter D of a stud 15 of another toy-building element 10.

A contact wall 22 forms together with two contact ribs 16 an opening in which a coupling stud 15 fits with an interference fit with three contact lines as shown in the upper portion of FIG. 4. A contact wall 22 forms together with one rib 16 and one contact wall 22 of another projection 20 an opening in which a coupling stud 15 fits with an interference fit with three contact lines as shown in the lower portion of FIG. 4. The contact lines that are not associated with a contact wall coincide with the ribs 16. The contact lines extend preferably parallel with the axis of the coupling studs 15 and extend even more preferably over the height h of the coupling studs 15.

For studs 15 that are not inserted into the center of a tube two of the three contact lines are angularly spaced by a 90° angle from one another around the coupling stud 15 with the third contact line being spaced by a 135° angle from each of the other two contact lines. For studs 15 that are inserted in the center of a tube there are four contact lines equally distributed at a 90° angle.

The wall thickness of the tube is varied along the circumference of the tube in order to obtain a homogeneously distributed stress level in the tube when a stud 15 is disposed in an opening bordering the tube concerned or in the tube concerned. The wall thickness of the tube is varied along the circumference of the tube in order to obtain maximum resiliency against deformation by the insertion of a stud 15.

The circular outline of the coupling studs 15 is slightly larger than the opening between the contact walls 22 and the ribs 16 on the inner side of the side walls 14.

FIGS. 5 to 7 illustrate another example embodiment of a toy-building element 10 that is essentially identical to the toy-building element illustrated with reference to FIGS. 1 to 4, except that the toy-building element 10 has only two by two rows of studs 15 on the top surface 13 and only one projection 20 in the interior of the toy-building element 10.

A toy-building element 10 with two by two rows of studs 15 and with four by two rows of studs in a squared arrangement has been shown in the above embodiments. However, the toy-building element 10 can be provided with a top face 13 with at least two by at least two rows of equidistantly spaced coupling studs 15 in a squared pattern, without any upper limit to the number of rows in each direction. The openings defined between the contact walls 22 ribs 16 on the side of the sidewalls facing the interior 19 are arranged in a pattern that matches the squared pattern of the studs 15 on the top face 13 and the number of projections 20 and their placement is adjusted accordingly.

In an embodiment, the pitch between the coupling studs 15 in the squared pattern is 8 mm. In an embodiment, the height H of a building element between the bottom face and the top face is 9.6 mm. In an embodiment, the length L of a building element with a rectangular top face and bottom face outline is a equal to the product of the number of coupling studs along the length of the building element times 8 mm, with 0.2 mm subtracted from the product to arrive at the length L. In an embodiment, the width W of a building element with a rectangular top face and bottom face outline is a equal to the product of the number of coupling studs along the width of the building element times 8 mm, with 0.2 mm subtracted from the product to arrive at the width W. In an embodiment, the coupling studs have a diameter D of 4.9 mm.

In the example embodiment of FIGS. 9 and 10, each contact wall 22 is provided with a contact rib 17 extending from the end of the contact wall 22 at the bottom face towards the top face, a contact rib 17 on a contact wall 22 preferably being arranged to form the contact surface with a stud 15 from another toy-building element 10. In this embodiment, the ribs form the contact surface of the contact walls 22 for contact with a stud 15 of another toy-building element 10.

The toy-building elements 10 are manufactured by a method that includes injection molding the toy-building element 10 in a mold (not shown) that comprises at least 2 mold parts, where one of the mold parts comprises a mold core which shapes the inner faces of the toy-building element 10, wherein the mold core comprises a recess to form the at least one projection 20.

The toy-building element 10 is in the shown the Figs. is illustrated as a brick 10, but could just as well be a thinner element, i.e. with the lower height H, such as e.g. toy-building element resembling a plate.

The invention has been described in conjunction with various embodiments herein. However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. The reference signs used in the claims shall not be construed as limiting the scope. 

1.-12. (canceled)
 13. A toy-building element for a toy-building system in which toy-building elements can be releasably coupled to one another using an interference fit between coupling studs of one toy-building element inserted in matching openings in another toy-building element, the toy-building element comprising: a body part with a top face, a bottom face and one or more flat sidewalls that extend around an outer periphery of the toy-building element, the one or more flat sidewalls at least partially define said interior of said toy-building element and the flat sidewalls extend between the top face and the bottom face with the sidewalls being connected to one another at right angles to form sharp corners, at least one cylindrical coupling stud extending at a given height from the top face, and at least one tubular projection extending inside the interior to the bottom face for forming at least one opening in the bottom face in which a coupling stud fits with said interference fit, said at least one projection being detached from said flat sidewalls, wherein said sides of said sidewalls facing the interior can be provided with contact ribs that extend from said bottom face towards said top face, characterized in that: the at least one tubular projection comprises four substantially flat contact walls extending perpendicularly to the bottom face, said four contact walls being arranged in a squared arrangement with each of the four contact walls arranged at a 45° angle with said side walls, said four contact walls of a tubular projection are connected to one another to form a tube with a squared cross-sectional outline, and in that a contact wall forms together with two sidewalls or with two contact ribs an opening in which a coupling stud fits with an interference fit with three contact lines and/or wherein a contact wall forms together with one sidewall or one contact rib and one contact wall of another tubular projection an opening in which a coupling stud fits with an interference fit with three contact lines.
 14. A toy-building element according to claim 13, wherein the four contact walls of one tubular projection together define an opening in which a coupling stud fits with an interference fit.
 15. A toy-building element according to claim 13, wherein said cross-sectional outline of said tube resembles or equals a rounded square, a square with rounded corners or a squircle.
 16. A toy-building element according to claim 14, provided with at least two of said tubes with neighboring tubes being interconnected with one another at opposing corners of said tubes concerned.
 17. A toy-building element according to claim 16, provided with at least two of said tubes with neighboring tubes being interconnected with one another at opposing corners of said tubes concerned and the other corners of said tubes are not interconnected with anything, wherein said wall thickness of said tubes concerned at said interconnected corners is reduced relative to said wall thickness of said other corners of said tubes concerned.
 18. A toy-building element according to claim 17, wherein said wall thickness of said tube is varied along said circumference of said tube in order to obtain a homogeneously distributed stress level in said tube when a stud is disposed in an opening bordering said tube concerned or in said tube concerned.
 19. A toy-building element according to claim 17, wherein said wall thickness of said tube is varied along said circumference of said tube in order to obtain resiliency against deformation by said insertion of a stud.
 20. A toy-building element according to claim 18, wherein said wall thickness along said circumference of said tube is higher at said corners of said tube and higher in said middle of said extent of said tube between corners and lower in said remaining circumferential extent of said tube.
 21. A toy-building element according to claim 13, wherein the at least one cylindrical coupling stud having a given outer diameter, and wherein said size of said gap between opposing contact walls of a tubular projection is slightly less than the diameter.
 22. A toy-building element according to claim 13, wherein said circular outline of said coupling studs is slightly larger than said opening between said contact walls and said side walls or said ribs on said inner side of said side walls.
 23. A method for manufacturing a toy-building element according to claim 13, wherein said method comprises injection molding said toy-building element in a mold that comprises at least two mold parts, where one of said two mold parts comprises a mold core, which shapes said inner faces of said toy-building element, and wherein said mold core comprises a recess to form said at least one tubular projection.
 24. A molding tool for the manufacture of a toy-building element according to claim 13, wherein said mold comprises at least two mold parts, where one of said mold parts comprises a mold core, which shapes said inner faces of said toy-building element, and wherein said mold core comprises a recess to form said at least one tubular projection. 